The present invention relates to a brushless motor which is constituted by a rotor having a plurality of poles, a stator having a plurality of slots and making three-phase connection to the slots, and a driving circuit for applying three-phase full-wave current to the stator, and concerns an electric appliance such as an air conditioner, an air cleaner, and a water heater, or a vehicle that is equipped with the motor.
In recent years, lower noise and lower vibration of brushless motors have been in increasing demand. As shown in FIG. 8, conventionally, a brushless motor has been constituted by an eight-polar rotor 1, a stator 3 which has twelve slots 2 and making three-phase connection to the slots 2, and a driving circuit 5 which applies 120xc2x0 three-phase full-wave current to the stator 3. In this way, the ratio of the number of poles on the rotor 1 to the number of slots on the stator 3 has been 2:3.
In general, it is said that when current applied to a winding to reduce torque pulsation is brought close to a sine wave, noise and vibration can be effectively reduced. Since voltage applied to a winding is determined by a difference between a line induced voltage and a power supply voltage, when line induced voltage is closer to a sine wave, noise and vibration can be reduced more effectively.
However, in the conventional brushless motors, line induced voltage has a poor distortion factor, resulting in problems of noise and vibration.
In FIG. 8, voltage f8(xcex8), which is induced by a conductor group 12a on one side of one tooth, is expressed below by Fourier expansion.                                           f            8                    ⁡                      (            θ            )                          =                              ∑                          n              =              1                        ∞                    ⁢                                    a              u                        ⁢            sin            ⁢                          {                                                (                                                            2                      ⁢                      n                                        -                    1                                    )                                ⁢                θ                            }                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
In the above equation, xcex8 represents an electrical angle (xc2x0).
A line induced voltage f8(xcex8) is the sum of voltages induced by conductor groups 12a to 12p and is obtained by adding 16 phase differences of the (equation 1) as shown below.
F8(xcex8)=4f8(xcex8)+8f8(xcex8+60xc2x0)+4f8(xcex8+120xc2x0)xe2x80x83xe2x80x83(Equation 2)
The equation (2) is expressed for each high-frequency component as shown below.
F8(xcex8)=12a1sin (xcex8+60xc2x0)+12a3sin (5xcex8+300xc2x0)+12a4sin (7xcex8+60xc2x0)+xe2x80x83xe2x80x83(Equation 3)
Hence, when the ratio of the number of poles on the rotor to the number of slots on the stator is 2:3, although tertiary components of line induced voltage can be reduced, quintuple and septenary components cannot be reduced. FIG. 9 shows a line induced voltage waveform at 1000 r/min. Since only a sine wave with a distortion factor of 2 to 5% is obtained, torque pulsation occurs, resulting in problems of low noise and low vibration. Since the 120xc2x0 three-phase full-wave current-carrying method is adopted, when phases of two-phase current application are switched, current applied to a coil largely fluctuates and torque pulsation occurs, which is a disadvantage against low noise and low vibration.
The present invention in intended to solve the above-mentioned conventional problem, and has an object to provide a low-noise and low-vibration brushless motor and a more quiet electric appliance and a vehicle that are equipped with the brushless motor, by optimizing a ratio of the number of poles on a rotor to the number of slots on a stator, the ratio being suitable for a driving circuit.
In order to solve the above-mentioned problem, the present invention provides a brushless motor which comprises a rotor having a plurality of poles, a stator having a plurality of slots and making three-phase connection to the slots, and a driving circuit for applying three-phase full-wave current to the stator, the motor being characterized in that the ratio of the number of poles on the rotor to the number of slots on the stator is 10:12, and the conduction width of the driving circuit is set at an electrical angle of 135 to 180xc2x0.
Further, the brushless motor is mounted in the electric appliance and a vehicle.